Morphological conservatism and variability in the ephippium sculpture of the genus Simocephalus Schoedler, 1858 (Crustacea: Cladocera: Daphniidae)

Abstract

Despite the important advances made in the taxonomy of water fleas (Crustacea: Cladocera), many taxa have not yet been revised. The genus Simocephalus Schoedler, 1858 (Anomopoda: Daphniidae) belongs to understudied groups. During the last forty years, several attempts have been made to use genetic approaches to estimate species diversity within Simocephalus. At the same time, investigations on morphology almost stopped after the last revision of Orlova-Bienkowskaja (2001). The lack of morphological data, even on the most widespread taxa, makes it difficult to apply the remains of this group to paleoecological reconstructions, although Simocephalus ephippia are common in subfossil sediments and fossils of different geological ages. We investigated more than 80 ephippia of recent representatives of the genus from Eurasia and North America, and 45 ephippia from Pleistocene sites of different ages on these two continents. For the first time, we have distinguished three main morphotypes based on the sculpture of the ephippia among the recent representatives. Ephippia of the vetulus-type are covered by polygonal hillocks of moderate height and with prominent processes. This morphotype is the most common in Eurasia, but it could be found on other continents, considering the distribution patterns of the S. vetulus species complex. Ephippia of the vetuloides-mixtus-type are covered by hillocks with processes strongly protruding above the ephippium surface. This morphotype is found only in the Asian part of Russia; it has more localized distribution than the vetulus-type. Ephippia of the serrulatus-type are covered by almost flat hillocks, or with hillocks of moderate height. This morphotype is found in both Eurasia and North America, which may confirm previous genetic data on a close relationship between serrulatus populations from the Eastern Palearctic and North America. Ephippia of the vetulus- and serrulatus-types are found in several Pleistocene localities. Due to lack of information about ornamentation patterns of Simocephalus ephippia of pre-Cenozoic age, we could not estimate the precise time of the morphotype separation. However we may conclude that in the Late Pleistocene the vetulus- and serrulatus-types were broadly distributed in Northern Eurasia and North America. Simocephalus ephippia demonstrate a morphological stasis at least since the Late Pleistocene, or even during longer time. In Maly Chukochiy Cape deposits (Republic Yakutia) we found a sole ephippium presumably belonging to an extinct taxon of Simocephalus. Eastern parts of Northern Eurasia had diverse communities of Simocephalus at the Pleistocene-Holocene boundary, and these then changed analogously to the transformation of Daphnia communities. In the course of our work, we obtained a large set of illustrations of Simocephalus ephippium morphology in different populations. We hope that our study will attract more attention to the taxonomy of modern Simocephalus and its Pleistocene-earlier Holocene subfossil ephippia.